The present invention relates to a rotary sealing device for connecting a fixed fluid supply pipe to a tube rotating about its axis, particularly intended for use on an apparatus for the production of optical fibers.
The term optical fiber is generally understood to mean a light wave guide which, in its simplest form, consists of at least two layers of glass. One layer forms the core of the fiber and the other forms the cladding for the fiber and is placed round the core, whilst having a refractive index below that of the core. In certain applications, it is known to use a fiber in which the index varies radially in a continuous or discreet manner from a higher value at the center to a lower value at the periphery. Various processes are known for the production of such fibers and they generally comprise three main steps:
the production of a solid glass cylinder, called the preform, whose index varies discreetly or continuously from the center towards the periphery;
fiber formation, i.e. the drawing out of the preform in order to obtain a greater fiber length, whose diameter is well below that of the preform;
the protection of the thus obtained fibers.
A number of different processes for producing the preforms are also known and in particular a vapour phase deposition process known under the initials M.C.V.D. (Modified Chemical Vapor Deposition) consisting of using a hollow cylindrical tube, e.g. of silica, through which passes a gaseous stream of silicon chloride and oxygen, to which may be added a certain number of compounds (generally halides) of materials for doping the silica, in order to vary its index. The tube performs a rotary movement and is externally and locally heated in order to bring about an oxidation of the gases, as well as the deposition and vitrification of the products of the chemical reaction within the tube and at the heated point. The heating area is displaced all along the tube.
Such a production process leads to a sealing problem. Thus, the gas supply pipe is generally fixed, or may perform a translation movement, whereas the silica tube rotates. This sealing problem is particularly critical due to the vulnerability of the optical fibers to impurities and in particular the humidity contained in the air and which leads to variations in the chemical composition of the glass forming the preform. These variations result in selective energy absorptions as a function of the wavelength and particularly at about 1.4 .mu.m. This is particularly disadvantageous when using an optical fiber obtained from such a preform.
It is known to use a rotary joint between the gas supply pipe and the rotary silica tube to bring about such a seal. However, this solution gives rise to problems, linked with the fact that the silica tube diameter is not constant and the rotary joint is subject to wear due to the mechanical movement and to corrosion resulting from the gases used for producing the preform.